1. Field of the Invention
The present invention relates to an emergency cooling system for a component which is subject to thermal load in operation, in particular a component of a turbine. The invention also relates to a plug and to a component which are suitable for use in an emergency cooling system of this type.
2. Discussion of Background
Thermally loaded components are to be found, for example, in gas turbines. In particular, in gas turbines guide vanes, rotor blades and heat shields are exposed to flows of hot gases. On account of the temperatures of the hot gases which surround them, these components have to be cooled. One particular difficulty is that of reliably cooling certain regions of the components in question which have been particularly exposed to the thermal loading. One of these certain regions is, for example, a shroud or shroud element of the blade or vane and a cavity which is formed between fins of the shroud element. Intensive cooling is required here to reliably prevent overheating. Overheating at this location leads to oxidation and to deformation of the shroud element and therefore to a larger gap being formed between the thermally protective shield located opposite the turbine blade or vane and the turbine blade or vane itself. An enlarged gap leads to a greater quantity of hot gas flowing into the cavity and therefore to further overheating, with terminal consequences for the gas turbine. Cooling of the corresponding thermally loaded components, for example of a turbine component, is designed for a nominal operating point of the appliance fitted with this component, for example of a gas turbine, in order in this way to ensure the required cooling within this nominal operating point. Nevertheless, operating situations may arise in which the thermal load on the component in question exceeds the thermal load provided for the nominal operating state. However, for efficiency reasons, cooling is restricted to the extent required for the design point, in order to avoid energy-consuming, unnecessary cooling at the design point.
An air-cooled turbine blade or vane, which at its tip has a shroud element extending perpendicular to its longitudinal axis, is known from German patent application DE 102 25 264.5 on Jun. 7, 2002, which had not yet been published on the application date of the present patent application. This shroud element has at least one cooling-air hole passing all the way through it for cooling purposes, and on the inlet side this hole is in communication with at least one cooling-air passage which runs through the turbine blade or vane, while on the outlet side it opens out into the outer space which surrounds the turbine blade or vane. Inside the cooling-air hole there is a valve which opens as a function of the temperature of the outer space which surrounds it. This valve may be formed, inter alia, by a plug which consists of a material which melts as soon as a certain temperature is reached. The result of this is that during normal operation of the turbine blade or vane, the plug keeps the cooling-air hole closed and only opens it up when the tip of the turbine blade or vane threatens to overheat, i.e. in situations in which there is an extraordinarily high thermal load. In this way, it is possible to prevent the turbine blade or vane from overheating. This design therefore provides an emergency cooling system which, in the event of the thermal load on the component exceeding a predetermined limit, opens up an emergency cooling opening as a result of the plug melting, so that the cooling air can then pass through this opening into the overheated outer space. This results firstly in a drop in the mixing temperature in the vicinity of the component which is to be cooled, so that the thermal load on'this component is reduced, and secondly the cooling air blown out leads to an increase in pressure in the area surrounding the component which is to be cooled, with the result that the mass flow of hot gas acting on the component is reduced, which likewise lowers the thermal load on the component.
The abovementioned DE 102 25 264.5 does not describe how the plug can be introduced into the cooling-air hole. By way of example, it would be conceivable for the plug to be cast into the cooling-air hole while the turbine blade or vane in question is being produced. However, this procedure may make the subsequent replacement of a plug, which has melted out in the event of an emergency, a relatively complex operation.